Recently, a demand for a device capable of irradiating simulated solar light, that is, artificial light similar to solar light has been increased. Especially, solar battery technology has been advanced and popularized so fast. As a result, there is a high demand for a device capable of irradiating highly accurate simulated solar light usable in inspection, measurement, and experiment. A main aspect required for such simulated solar light is to have a luminescence spectrum similar to that of natural solar light.
Patent Literature 1 discloses one example of a simulated solar light irradiation device. This simulated solar light irradiation device is configured to include (i) a lamp housing which has a light irradiation surface having an area smaller than an irradiation range of simulated solar light, an optical filter installed on the light irradiation surface, a lamp such as xenon lamp installed inside the lamp housing, and (ii) a reflective plate provided to face the optical filter of the lamp housing, wherein an irradiation target on an appropriate mount is placed to face against a reflective surface of the reflective plate, the lamp is turned on to emit simulated solar light, which then passes through the optical filter so as to be reflected and diffused by the reflective plate, thereby irradiating the irradiation target with the simulated solar light thus reflected and diffused.
By configured as such, a simulated solar light irradiation device employing a diffused light irradiation technique can be downsized by having a smaller installation area for the optical filter, and have a longer diffusion optical path necessary for achieving uniform irradiation distribution.
However, the technique of Patent Literature 1 does not control directivity (radiation angle) of the light passing through the optical filter. Because of this, an optical filter made from a multilayered film and having an incident angle dependency can not fully exercise its capacity when the optical filter is adopted in the technique of Patent Literature 1. This leads to such a problem that simulated solar light having a desired luminescence spectrum cannot be emitted.
A light source device to solve this problem has been developed. Patent Literature 2 discloses a light guide applicable to various types of simulated solar light irradiation devices and a light source device provided with the light guide.
FIG. 12 is a schematic cross sectional view of a light source device described in Patent Literature 2. This device is configured such that a linear light source constituted by a fluorescent tube is placed inside a reflective box whose inner wall is covered with a silver film, and the reflective box has openings provided on a surface of the reflective box at constant intervals along the linear light source. Each opening is provided with the light guide tapered in shape, so that light enters the light guide from a smaller-area end surface and outgoes from a larger-area end surface. The light entering from the incident end surface reaches an outer wall of the light guide. Then, the light is totally reflected repeatedly on the outer wall, thereby being gradually converted into a light perpendicular to the outgoing end surface, and finally being outputted from the outgoing end surface. By this, it is makes it possible to highly efficiently obtain outgoing light having a light directivity, irrespective of a light emission characteristics of the light source.
With this device, it is possible to obtain light with high directivity more highly efficiently by using a plurality of light guides. Moreover, the use of this light source device makes it possible to enter the directivity-controlled light into the optical filter, thereby making it possible to obtain simulated solar light having a desired luminescence spectrum.